The present invention relates to line card circuits and more particularly pertains to improved key telephone unit (KTU) line card circuits that prevent a clandestine listening device implanted in a subscriber station from utilizing the telephone line as a transmission path out of the subscriber's building.
A typical key telephone unit is generally used to permit a subscriber to have several different telephone lines, each line having a different number available for use with a single subscriber station. Such key telephones are recognizable by the usual line buttons thereon that may be individually operated by the user to select the telephone line to be used from among the several different lines connected to the subscriber station. For example, a particular telephone line would be selected and used by depressing the corresponding button at the subscriber station and placing the subscriber station in an off-hook condition.
As is well known, line card circuits are generally used in key telephone systems to control the operating condition of a telephone line and to control the operation of the audible and visual signaling devices at the subscriber station. A single line card circuit is installed at the location of the subscriber station for each different telephone line serving the subscriber station.
A telephone line is generally capable of being in one of three operating conditions. These conditions are the ringing condition, the seized condition and the hold condition. The ringing condition will exist during periods in which ringing current is being supplied to the subscriber station in response to a ringing indication from the central office. An audible signaling device, such as a ringer or buzzer, or the like located at the subscriber station being called will respond to such ringing current and provide an audible indication that an incoming call is being received. A light position beneath or in close proximity to the line button corresponding to the line on which the incoming call is being received will be illuminated to identify the telephone line at the subscriber station receiving the call. Such line button light will also be illuminated to identify the telephone line being used for an outgoing call.
A seized line exists during periods in which parties to a telephone conversation have their respective subscriber stations operated to permit the conversation to be carried on. A hold condition is generally used by a subscriber when it is desired to interrupt a conversation without releasing the telephone line being used. For example, a subscriber in the middle of a conversation on a first line may desire to answer an incoming call on a different line. The first line would be placed in a hold condition by operating a hold button at the subscriber station. The subscriber could then answer the incoming call without dropping the first line. Conversation on the held telephone line can later be resumed by simply operating (usually by depressing) the coresponding line button. Obviously, the hold condition permits several telephone lines to be served at once from a single subscriber station.
The tip and ring lines of each telephone line terminating at the subscriber station provide a convenient transmission path out of the subscriber's building for any clandestine listening device placed within the subscriber station for the purpose of overhearing conversations conducted in the room within which the subscriber station is located. Utilization of these types of listening devices would be discouraged if the telephone line were not such a convenient transmission path of the building in which the subscriber station is located.